The X-ray beam in most computer tomography (CT) scanners is generally polychromatic. However, third-generation CT scanners generate images based upon data according to the energy integration nature of the detectors. These conventional detectors are called energy-integrating detectors and acquire energy integrated X-ray data. On the other hand, photon-counting detectors are configured to acquire the spectral nature of the X-ray source, rather than the energy integrated nature. To obtain the spectral nature of the transmitted X-ray data, the photon-counting detectors split the X-ray beam into its component energies or spectrum bins, and count a number of photons in each of the bins. The use of the spectral nature of the X-ray source in CT is often referred to as spectral CT. Since spectral CT involves the detection of the transmitted X-ray at two or more energy levels, spectral CT generally includes dual-energy CT by definition.
Spectral CT is advantageous over conventional CT as it offers the additional clinical information inherent in the full spectrum of an X-ray beam. For example, spectral CT improves discriminating tissues and differentiating between materials such as tissues containing calcium and iodine, and enhances the detection of smaller vessels. Among other advantages, spectral CT is also expected to reduce beam-hardening artifacts and to increase accuracy in CT numbers independent of individual scanners.
Conventional spectral CT approaches include the use of the integrating detectors in implementing spectral CT. One attempt included dual sources and dual integrating detector units that are placed on the gantry at a predetermined angle with respect to each other for acquiring data as the gantry rotates around a patient. Another attempt includes a single source that performs kV-switching and a single integrating detector unit that are placed on the gantry for acquiring data as the gantry rotates around a patient. Yet another attempt includes a single source and dual integrating detector units that are layered on the gantry for acquiring the data as the gantry rotates around a patient. All of these attempts at spectral CT are not successful in substantially solving issues such as beam hardening, temporal resolution, noise, poor detector response, poor energy separation, etc. for reconstructing clinically viable images.
Conventional spectral CT approaches also include the replacement of the conventional integrating detectors by photon-counting detectors in implementing spectral CT. In general, photon-counting detectors are costly and have performance constraints under high flux. Although at least one experimental spectral CT system has been reported, the costs of high-rate photon-counting detectors are prohibitive for a full-scale implementation. Despite some advancement in photon-counting detector technology, currently available photon-counting detectors still require solutions to implementation issues, such as polarization due to space charge build-up, pile-up effects, scatter effects, spatial resolution, temporal resolution, and dose efficiency.